High performance metal, plastic, wood, and glass coatings are used in both indoor and outdoor applications, such as in building products, appliances, transports, etc. Due to increasingly more stringent emission rules and increasing energy costs associated with coating conversion and pollution controls, it is preferable that such coatings contain 100% solids and no volatile organic compounds (zero VOC's). Unfortunately, however, most of the non-polluting coating systems currently being used last only about 30 years.
Problems associated with the use of solids systems on a commercial scale include mediocre coating performance, slow line speed, difficulty in adopting the systems, and/or high initial capital cost. Furthermore, the coatings may not be fully non-polluting and may contain unacceptable VOC levels. For these reasons, solid systems have been unsuccessful in replacing solvent-borne or water-solvent borne systems in the high volume coil-coating industry.
However, the present state-of-the-art non-polluting coatings have their own set of shortcomings and problems. For example, even low VOC (1-3 lbs/gal) water-borne coatings contain some solvents that must be incinerated before being emitted to the atmosphere. With respect to solid powder coatings (zero VOC), associated handling problems often make them unsuitable for a high speed flat stock, coil-coating line. Furthermore, ultraviolet (U.V.) curable coatings, which are 100% solids, zero VOC, can be used only for non-pigmented coatings or thin film pigmented applications, such as for printing inks. In addition, the performance of U.V. curable coatings is in general inferior to that of thermally cured coatings in the areas of adhesion, U.V. stability, corrosion resistance, and weatherability, which are very important properties for use in building products and in automotive applications.
Thermal curing is also often unsuitable because of the impractical long oven length needed for the curing oven. A long oven is required because the thermal curing rate, i.e., conversion rate of the coating, for converting low viscosity monomers and oligomers to solid polymers is very slow. However, conversion to the solid state, which provides tack-free, dry coatings can be completed quickly by simply evaporating the solvent from the solid resin solution. Thus, the present state-of-the-art thermal systems employ this technique. Most thermal curing coating chemistries are based on solution, emulsion, or dispersion of solid resins or very high viscosity resins (high molecular weight thermoplastic or thermostat resins), which involve very little or no curing (solidification). However, one disadvantage associated with this type of solid/high viscosity resin is that the use of solvents or co-solvents is required, and thus, the materials contain VOC's. In addition, as is commonly known, low molecular weight, low viscosity monomers can also evaporate from the line, especially when heated at high temperatures and when present in thin films. Thus, even if the coatings meet the requirements under the present VOC definition, the coating line will still generate organic vapor emission of a different kind, i.e. from the monomers.
Electron beam (E-beam or E.B.) curable coatings, which contain 100% solids and zero VOC, can be used for pigmented coatings, unlike U.V. curable coatings. However, current E-beam coatings suffer from the same performance limitations as do U.V. curable coatings, when compared with thermal systems. This is largely because the same resin and curing chemistries (acrylates) are employed in existing E-beam curable coatings as in the U.V. materials. Additional costs incurred with radiation (both U.V. & E-beam) curable acrylate coatings include those associated with the use of nitrogen blanketing. However, of the currently available curing processes, E-beam irradiation is the most preferable for high speed coating lines when compared with thermal curing or U.V. curing.
Alternate available cationic curing (U.V. & E.B.) epoxy chemistry coating compositions do not require nitrogen blanketing, but the cure rates of currently available epoxies are very slow when compared with acrylates. In addition, a soft, tacky surface is left outside the area irradiated by the beam, which is unacceptable in a high speed, low dose line. Furthermore, various processing and performance limitations have made them undesirable for use in Original Equipment Manufacture (OEM) markets, such as in appliances, building products, and automotives, etc. Thus, until now, radiation curable (U.V. & E.B.) coatings have not been a good alternate technology for providing zero VOC, pollution-free coatings for coil coatings. Furthermore, current filled and pigmented coatings typically include the use of an environmentally unfavorable chromate filler in order to pass corrosion testing.
Therefore, a need exists for a cationic curable epoxy resin coating chemistry, which meets the process and performance parameters of coil coatings in OEM markets. Preferably, the viscosities of the monomers and oligomers should be low enough to formulate a highly filled and/or pigmented coating (up to 65% by weight) without the use of any solvents, and the coating should also be able to meet the application viscosity (less than 3000 cps) of a high speed reverse roll coating system, i.e. greater than 400 FPM.
In addition, it is desirable that certain economic and performance parameters be met. Thus, the resin chemistry of such coil coatings should be fully compatible with commonly used pigments and fillers, thereby providing formulations with minimum restrictions. It would also be environmentally advantageous if chromate-containing pigments could be eliminated as an ingredient.
Furthermore, to be useful in coating metal, plastic, wood, and glass substrates, the coatings should meet industry standards for each of the application areas in terms of adhesion, flexibility, gloss, weathering, corrosion, etc. Also, the coating chemistry should be suitable for high speed, low dose E-beam cure lines and alternatively, for thermal curing, such that the materials can be immediately rolled into coils without any coating lift up problem, with or without nitrogen blanketing.